The backing for most magnetic recording tapes is biaxially oriented polyester film, especially polyethylene terephthalate which has such qualities as a smooth surface, high tensile strength and toughness, good dimensional stability, good resistance to moderately elevated temperatures and unchanging properties with the passage of time. When used as a magnetic recording tape backing, polyester film does have a troublesome drawback. Low-molecular-weight organic components of the film, generally referred to as cyclic oligomers, principally trimers, extracted by the solvent used in coating the magnetizable coating, tend to migrate to and crystallize at the surface of the magnetizable coating. Even when the magnetizable coating is applied without solvent, there is a problem of migration of the low-molecular-weight components through the magnetizable coating over a period of time. Such components gradually transfer to and build up on the recording/playback heads to cause an undesirable loss of signal.
In a magnetic recording tape having a backside coating, such as a controlled-wind coating primarily comprising conductive carbon black and binder, the low-molecular-weight components also tend to be solvent-extracted into and to migrate to the surface of that coating. During storage of the tape in roll form, they tend to crystallize on the surface of the magnetizable coating as well as on the backside coating.
It is common to treat the polyester film backing to improve the adhesion of magnetizable coatings. For example, the polyester film backing may be treated with para-chlorophenol which attacks its surface, thus enhancing the adhesion of the magnetizable coatings. See U.S. Pat. No. 3,607,354. However, this adhesion-promoting treatment seems to aggravate the trimer exudation problem. U.S. Pat. No. 3,215,554 discloses vinylidene chloride/acrylonitrile copolymers as priming or subbing layers for a polyester film backing of magnetic recording tape. The subbing layer for the polyester film backing of the magnetic recording tape of U.S. Pat. No. 3,387,995 is a polycondensate of terephthalic acid, isophthalic acid and ethylene glycol. That of U.S. Pat. No. 3,661,874 is an aminized reaction product of an epoxidized polybutadiene. Of such subbing layers, it is believed that only that of U.S. Pat. No. 3,661,874, if thoroughly cured, provides a significant barrier to the migration of low-molecular-weight components from the polyester film. However, the temperature required for a good cure tends to distort the polyester film backing unless it is restrained during the cure.
An entirely different problem in the manufacturing of magnetic recording tape arises out of a desire that the surface of the magnetizable coating be as smooth as possible. Ordinarily, the surfaces of a polyester film are roughened by a slip agent for windup purposes, and that roughness is carried through in part to the surface of the magnetizable layer. A smoother magnetizable coating tends to exhibit reduced noise.